Effect of alkali treatment on the wettability and tensile failure of twisted bamboo fiber/epoxy composites
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摘要: 采用加捻竹纤维(TBF)为增强相、环氧树脂-酸酐体系为基体相，制备加捻竹纤维/环氧树脂(TBF/EP)复合材料，通过改变NaOH溶液浓度(1wt%~5wt%)，研究碱处理对TBF/EP复合材料润湿性和拉伸失效的影响。采用SEM、表面张力测试、原位加载等纳米和微观试验手段，对纤维-树脂结合状态、润湿性能和拉伸力学特性进行了分析。结果表明：碱处理降低了纤维表面能和极性，使TBF与基体润湿力从0.45 mN降至0.1 mN；3wt%NaOH溶液改性的TBF/EP复合材料含胶量降低至62%，拉伸强度(TS)达到273.70 MPa，比未处理复合材料提高178.64%；原位分析显示，TBF失效过程包括纤维断裂和纤维间滑移，而TBF/EP复合材料失效过程包括基体剪切屈服和纤维断裂，且随着浸润性提高，BF抑制屈服的效果增加。因此，TBF/EP复合材料的强度主要来源于纤维和界面的增强，受TBF与基体的浸润性、应力传递效果的影响。Abstract: The twisted bamboo fiber (TBF) was used as the reinforcement phase and the epoxy resin-anhydride system as the matrix phase to prepare the twisted bamboo fiber/epoxy resin (TBF/EP) composite. The effects of alkali treatment on the wettability and tensile failure of TBF/EP composites were investigated by varying the concentration of NaOH solution (1wt%-5wt%). Nano-scale and micro-scale experimental techniques, such as SEM, surface tension testing, and in-situ loading, were employed to analyze the fiber-resin interface, wetting properties, and tensile mechanical properties of the composites. The results show that alkali treatment reduces the surface energy and polarity of the fibers, resulting in a decrease in the wetting force between TBF and the matrix from 0.45 mN to 0.1 mN. The TBF/EP composite modified with 3wt%NaOH solution exhibits a tensile strength (TS) of 273.70 MPa, which is 178.64% higher than that of the untreated composite. In-situ analysis reveals that the failure process of TBF involves fiber fracture and fiber sliding, while the failure process of the TBF/EP composite includes matrix shear yielding and fiber fracture. Moreover, as the wetting properties improve, the inhibitory effect of the fibers on matrix yielding increase. Therefore, the strength of the TBF/EP composite is mainly derived from the reinforcement of the fibers and the interface, which is influenced by wetting properties and stress transfer effect between TBF and matrix.
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Key words:
- bamboo fiber /
- twisted structure /
- composite /
- wettability /
- failure mechanism
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图 1 加捻竹纤维(TBF)的制备过程：(a) 竹材；(b) 竹条；(c) 竹纤维束；(d) 竹纤维(BF)；(e) 加捻设备；(f) TBF；(g) TBF的SEM图像；(h) TBF制备示意图
Figure 1. Preparation process of twisted bamboo fiber (TBF): (a) Bamboo; (b) Bamboo strips; (c) Bamboo fiber bundles; (d) Bamboo fiber (BF); (e) Equipment of twisting; (f) TBF; (g) SEM image of TBF; (h) Schematic diagram of preparation of TBF
图 2 TBF/环氧树脂(EP)复合材料的制备工艺
Figure 2. Preparation process of TBF/epoxy resin (EP) composite
MeTHPA—Methyl tetrahydrophthalic anhydride
图 3 TBF的润湿性测试模型
Figure 3. Wettability test model of TBF
θF1, θF2, θF3, θF4, θF5—Different infiltration angles of the bamboo fiber units of TBF
图 4 TBF的SEM图像：((a)~(c)) TBF；(d) 1wt%-TBF；(e) 2wt%-TBF；(f) 3wt%-TBF；((g)~(i)) 5wt%-TBF (NaOH溶液浓度：1wt%、2wt%、3wt%、5wt%)
Figure 4. SEM images of TBF: ((a)-(c)) TBF; (d) 1wt%-TBF; (e) 2wt%-TBF; (f) 3wt%-TBF; ((g)-(i)) 5wt%-TBF (NaOH solution concentrations: 1wt%, 2wt%, 3wt%, 5wt%)
图 5 TBF/EP复合材料形态分析：((a)~(c)) 表面形态；((d)~(f)) 横截面；((g)~(i)) 横截面的黑白二值图
Figure 5. Morphological analysis of TBF/EP composites: ((a)-(c)) Surface morphology; ((d)-(f)) Cross section; ((g)-(i)) Black and white binary graph of cross section
图 7 (a) TBF在正己烷下的润湿力；(b) BF和TBF的表面能
Figure 7. (a) Wetting force of TBF under the n-hexane; (b) Surface energy of BF and TBF
ΔSE—Difference of surface energy between BF and TBF; FF, θF—Balance force and contact angle during falling; FR, θR—Balance force and contact angle during rising
图 9 TBF的原位拉伸失效分析：((a), (b)) 拉伸力学；((c)~(f)) 0.05、0.10、0.20、0.22 mm时TBF拉伸形貌
Figure 9. In-situ tensile failure analysis of TBF: ((a), (b)) Tensile mechanics; ((c)-(f)) Tensile morphologies of TBF at 0.05, 0.10, 0.20 and 0.22 mm
图 10 TBF/EP复合材料原位拉伸失效分析：((a), (b)) 拉伸强度；((c)~(f)) 0.05、0.10、0.24、0.25 mm时3wt%-TBF/EP复合材料拉伸形貌
Figure 10. In-situ tensile failure analysis of TBF/EP composite: ((a), (b) ) Tensile strength; ((c)-(f)) Tensile morphologies of 3wt%-TBF/EP composite at 0.05, 0.10, 0.24 and 0.25 mm
表 1 TBF的基本性质
Table 1. Basic properties of TBF
BF Length/cm Width/mm Thickness/mm Strength/MPa 10 0.36-0.7 0.13-0.2 180±16 TBF Diameter/mm Tensile/N Linear density/(g·m−1) Twist angle/(°) 1.28±0.12 48.8 12 64 
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表 2 加捻竹纤维的接触角和表面能
Table 2. Contact angle and surface energy of TBF
Index Wetting boundary/mm Advancing contact angle/(°) Owens-Wendt worth/mN Surface energy/mN Water Ethylene glycol Diiodomethane Dispersion Polar 0wt% 21.94 61.75 48.56 40.48 22.49 10.23 32.72 1wt% 11.46 61.67 41.96 44.34 22.73 8.51 31.24 2wt% 12.86 61.91 46.12 47.19 23.72 7.58 31.30 3wt% 10.42 64.76 45.70 54.58 21.00 7.09 28.08 5wt% 11.26 69.91 47.30 59.01 22.03 4.21 26.23
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